Data driven keeper for a domino circuit

ABSTRACT

A domino circuit may be provided with additional keeper transistors that are selectively activated when one of the input transistors in a logic structure has a low or inactive signal applied to it during the evaluation stage. Thus, the potential of the output node of the domino circuit may be maintained, improving the soft error rate.

BACKGROUND

[0001] This invention relates generally to domino circuits.

[0002] A static CMOS gate is a fully complementary logic gate with P and N-channel (i.e., p-channel and n-channel metal oxide semiconductor) devices configured to implement a desired logic function. A dynamic complementary metal oxide semiconductor (CMOS) gate includes an N-channel device logic structure having a pre-charge or output node precharged to the supply voltage with a single clocked P-channel device. The output node is conditionally discharged (evaluated) by a set of devices forming the logic structure, coupled to external ground.

[0003] The clocked P-channel device has its gate coupled to an input clock signal. When the input clock signal is active, the output node is “precharged” through a P-channel device to the supply voltage. When the clock input is inactive, the output node is conditionally discharged (evaluated) through the set of devices forming the logic structure, to external ground. The logic structure may implement a logic function such as a NAND or a NOR logic function as examples.

[0004] Dynamic or domino logic circuits are dynamic because operation of the circuit is controlled dynamically by an input clock signal. Domino logic units are typically arranged in a plurality of domino stages, each stage having logic cells, such as NAND gates or NOR gates as examples, with each stage separated by an inverting stage. In this arrangement, an input signal applied to the first stage while the clock signal is active, triggers operation of the remaining stages in sequence. This yields a domino-like signal propagation effect within the logic unit.

[0005] During the evaluation phase, the inputs to the set of devices forming the logic structure only change from non-active to an active state. Otherwise, the output node may be corrupted without P-channel devices to pull the output node back up. Inverting stages are provided between each logic stage to facilitate proper precharging or evaluating of the individual domino circuits active during the precharge phase.

[0006] Referring to FIG. 4, a single stage NAND gate domino circuit 10 includes a single P-channel device 12 in combination with a set of N—channel devices 14 forming a logic structure. A keeper 16 may include an inverter and a P-channel transistor. The keeper offsets any charge leakage that might occur at the output node when the N-channel devices 14 of the logic structure are inactive. A domino NAND gate may be referred to as a “clocked” domino gate because the input clock signal is connected to an N-channel device in series with the N-channel devices of the logic structure. The single N-channel device connected to the clock signal selectively blocks a power dissipation path between the external power supply and external ground during the precharge phase.

[0007] In use, dynamic or domino logic circuits operate in phases including a precharge phase and an evaluate phase. During the precharge phase, the transistors of the logic structure of the domino circuit are precharged. During the evaluate phase, input signals may be applied to the gates of each of the transistors of the logic structure and the clock signal is active.

[0008] As feature size and external power supplies are scaled as a result of advances in CMOS process technology, a significant decrease in the charge at the input and output nodes of domino circuits may result. This decrease in node charge may lead to an increase in soft error rates (SER) caused by cosmic radiation and packaging materials.

[0009] During an SER event, domino circuits may generate errors while holding a high or active level at the precharge node. Noise immunity may be improved by adding a P-channel keeper. Although the robustness of a domino gate against soft errors may be enhanced by increasing the size of the keeper, this eventually leads to performance degradation. This may be due in part because contention may arise between the keeper and the N-channel devices of the logic structure. This degradation may be significant, in large fan-in domino topologies for example, as the effective width of the N-channel devices of the structure is small.

[0010] Thus, there is a continuing need for better ways to decrease the soft error rates of domino circuits.

SUMMARY

[0011] In accordance with one aspect, a domino circuit includes an output node and an input transistor coupled to the output node. A charge source is coupled to the node and the transistor to selectively charge the node depending on the signal applied to the transistor.

[0012] Other aspects are set forth in the accompanying detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic depiction of one embodiment of the present invention;

[0014]FIG. 2 is a schematic depiction of another embodiment of the present invention;

[0015]FIG. 3 is a schematic depiction of another embodiment of the present invention; and

[0016]FIG. 4 is a depiction of a corresponding structure in accordance with the prior art.

DETAILED DESCRIPTION

[0017] Referring to FIG. 1, a domino circuit 40 includes a P-channel transistor 42 having its source coupled to an external power supply potential. The drain of the transistor 42 is coupled to the output node 44. A set of three N-channel input transistors 46, 48 and 50 form a logic structure 51. The devices 46, 48 and 50 are illustrated as being coupled to the drain of the P-channel device 42. In one embodiment of the present invention, the logic structure 51 is a NAND gate formed of N-channel transistors. An additional N-channel transistor 52 has its source coupled to external ground and its gate connected to a clock signal that also drives the gate of the P-channel transistor 42. The drain of the transistor 52 is coupled to the logic structure 51, and in the illustrated embodiment, the source of the input transistor 50.

[0018] While a logic structure 51 that is a NAND gate having three input transistors is illustrated, a variety of other logic structures may be implemented using the principles set forth in the present invention. Another logic structure normally implemented with domino circuits is NOR gate as an example.

[0019] When the clock signal (CLOCK) is active or low, the output node 44 is charged up and is prevented from being discharged because the transistor 52 is not conducting. This condition generally corresponds to the precharge state of the domino circuit 40.

[0020] Each input transistor 46, 48 and 50 has its gate coupled to receive an input signal indicated as I1, I2 or I3. Each input signal is also coupled to a P-channel transistor 54, 56 or 58, each arranged to act as a keeper device. Each P-channel transistor 54, 56 and 58 has its drain coupled to the output node 44 and its source coupled to the external supply voltage. In one embodiment of the present invention, the drains of adjacent pairs of P-channel transistors, such as the transistors 54 and 56, may share drain diffusions as indicated in dashed ovals in FIG. 1.

[0021] Also coupled to the output node 44 is an inverter 60 and a P-channel transistor 62 that form a keeper circuit. Like the transistors 54, 56 and 58, the transistor 62 is also coupled between the external supply voltage and the output node 44.

[0022] As an example, where the input transistors 46, 48 and 50 are eight microns in width, and the transistor 52 is also eight microns in width, the effective width of the combined transistors is two microns. In this case, the transistors 54 to 62 may be relatively smaller devices, each have a width of about one micron for example. However, as can been seen in FIG. 2, due to the use of a plurality of transistors 54, 56, 58 and 62, the node 44 charge sustaining ability may be increased.

[0023] When all of the input signals I1, I2 and I3 are high, the transistors 54, 56, and 58 are all shut off and thus the transistors 54, 56 and 58 do not deteriorate the delay. However, when one or more of the input signals I1, I2 or I3 is inactive or low, the domino circuit 40 does not evaluate and one or more of the P-channel transistors 54, 56 and 58 is enabled or active. Thus, the transistor 54, 56 or 58 coupled to an inactive input signal tends to sustain the potential on the output node 44. For example, if I2 is inactive or low, P-channel transistor 56 may be active. By sustaining the potential on the output node 44, the soft error rate may be improved.

[0024] In some embodiments of the present invention, the effective keeper strength (which is a result of the transistors 54, 56, 58 and 62) may be increased two to four times compared to the design shown in FIG. 2. This domino technology may be used effectively in address decoders and particularly in situations where domino circuits are driven directly from latches.

[0025] Thus, in the circuit 10, the transistors 54, 56 and 58 act as data driven keepers. That is, they selectively sustain the potential on the node 44 depending on the state of the input signals I1, I2 and I3. Where the input signal to a given transistor in the logic structure 51 is low, a keeper transistor 54, 56 or 58 coupled to that transistor's gate may actively supply charge to the node 44.

[0026] While the present invention is illustrated as using n-channel transistors in the logic structure, n-channel transistors may be used in place of p-channel transistors and vice versa.

[0027] The principles described in connection with a NAND gate domino circuit can also be applied to an exclusive OR (XOR) domino circuit as shown in FIG. 2. In this case, a p-channel transistor 66 is coupled to an output node 74. A transistor 68 is also coupled to the output node 74 and has the signal B coupled to its gate. Anther transistor 70 has an input signal A coupled to its gate. A clock signal is coupled to the gate of the p-channel transistor 66 and an n-channel transistor 72 which is also coupled to ground. A second pair of transistors 88 and 90 have the input signals B and A coupled to their gates. Each of the transistors 68, 70, 80 and 90 also have their gates coupled to a p-channel transistor 76, 78, 80 or 82 as illustrated. The drains of the transistors may share diffusions as indicated in dashed lines. A keeper transistor 84 has its gate coupled to an inverter 86 as described previously.

[0028] In this case, the transistors 76, 78, 80 and 82 act as data driven keepers. That is, they selectively sustain a potential on the node 74 depending on the state of the input signals A, B, {overscore (B)} and {overscore (A)}. Where the input signal to a given transistor in the logic structure is low, a keeper transistor 76, 78, 80 or 82 coupled to that transistors gate may actively supply charge to the node 74.

[0029] Referring next to FIG. 3, an embodiment of the present invention in connection with a NOR domino gate receives the input signals I1, I2 and I3 on the gates of transistors 96, 112 and 114. These gates are also coupled to keeper p-channel transistors 102, 104 and 106. In this case, the p-channel transistors 102, 104 and 106 are connected in series between a supply voltage and an output node 100. Also coupled to an output node 100, is an inverter 110 and a keeper p-channel transistor 108. A clock input signal in connected to the gate of a p-channel transistor 94 and an n-channel transistor 98. Again, the p-channel keeper transistors 102, 104 and 106 maintain the potential on the output node 100 when an input signal to a given transistor 96, 112 or 114 is low by actively supplying charge to the node 100.

[0030] While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention. 

What is claimed is:
 1. A domino circuit comprising: an output node; an input transistor coupled to said node; and a charge source coupled to said node and said transistor to selectively charge said node depending on the signal applied to said transistor.
 2. The circuit of claim 1 including a plurality of input transistors forming a logic structure.
 3. The circuit of claim 2 wherein said logic structure is a NAND gate.
 4. The circuit of claim 1 wherein the gate of said input transistor is coupled to the gate of a keeper transistor.
 5. The circuit of claim 1 including a plurality of input transistors and a plurality of keeper transistors, each input transistor coupled to a different keeper transistor.
 6. The circuit of claim 5 wherein said input transistors are N-channel transistors and said keeper transistors are P-channel transistors.
 7. The circuit of claim 6 wherein said input transistors are wider then said keeper transistors.
 8. The circuit of claim 1 including a plurality of input transistors and a plurality of keeper transistors, each input transistor having a gate coupled to the gate of a keeper transistor, at least one pair of keeper transistors having a common drain diffusion.
 9. The circuit of claim 8 wherein said keeper transistors are P-channel transistors having their sources coupled to a supply potential and their drains coupled to said output node.
 10. The circuit of claim 1 including a P-channel transistor having its gate coupled to a clock signal and its drain coupled to said output node, the source of said P-channel transistor coupled to a supply voltage, said domino circuit including an N-channel transistor having its drain coupled to said input transistor and a source coupled to external ground, the gate of said N-channel transistor being coupled to said clock signal.
 11. A method of operating a domino circuit comprising: detecting a signal applied to a gate of an input transistor in a domino circuit logic structure; and in response to said signal, supplying additional charge to the output node of said domino circuit.
 12. The method of claim 11 including coupling said input transistor to a keeper transistor.
 13. The method of claim 11 including causing a keeper transistor to conduct when the input signal to an input transistor is low.
 14. The method of claim 13 including forming at least two N-channel input transistors and forming at least two P-channel keeper transistors.
 15. The method of claim 14 including using a common drain for said keeper transistors.
 16. A domino circuit comprising: an output node; a logic structure; a keeper circuit that supplies charge to said output node in response to said logic structure receiving an inactive signal.
 17. The circuit of claim 16 wherein said logic structure includes a plurality of input transistors.
 18. The circuit of claim 17 wherein said logic structure is a NAND gate.
 19. The circuit of claim 16 including a plurality of input transistors and a plurality of keeper transistors, each input transistor coupled to a different keeper transistor.
 20. The circuit of claim 16 including a plurality of input transistors and a plurality of keeper transistors, each input transistor having a gate coupled to the gate of a keeper transistor, at least one pair of keeper transistors having a common drain diffusion. 